Not really explicit, but there's Art. 6 of the RSTP:
6 Registration periods
1.
Players may only be registered during one of the two annual registration
periods fixed by the relevant association. As an exception to this rule, a
professional whose contract has expired prior to the end of a registration
period may be registered outside that registration period. Associations are
authorised to register such professionals provided due consideration is given to
the sporting integrity of the relevant competition. Where a contract has been
terminated with just cause, FIFA may take provisional measures in order to
avoid abuse, subject to article*22.
2.
The first registration period shall begin after the completion of the season and
shall normally end before the new season starts. This period may not exceed
12*weeks. The second registration period shall normally occur in the middle of
the season and may not exceed four weeks. The two registration periods for
the season shall be entered into TMS at least 12*months before they come into
force (cf.*Annexe*3, article*5.1 paragraph*1). FIFA shall determine the dates for
any association that fails to communicate them on time.
3.
Players may only be registered*– subject to the exception provided for in
article*6 paragraph*1*– upon submission of a valid application from the club to
the relevant association during a registration period.
4.
The provisions concerning registration periods do not apply to competitions
in which only amateurs participate. The relevant association shall specify
the periods when players may be registered for such competitions provided
that due consideration is given to the sporting integrity of the relevant
competition.
Source:
http://resources.fifa.com/mm/docume...tusandtransferofplayersapril2015e_neutral.pdf
So there's actually no registration of players allowed other than within the two registration periods with the one exception in 6.1. There's also no enabling provision for domestic asociations to make exceptions from this rule.
And regarding that 6.1 includes a very specific exception from this rule already, one could make a valid argument that the makers of this rule intended that there shouldn't be any further exceptions. Imo, the legal situation is unambiguous in this case.